1. Field of the Invention
The present invention relates to a cam grinding machine for grinding a cam of a workpiece, such as a cam shaft, which is capable of controlling a servomotor to rotate the cam at a constant circumferential speed.
2. Description of the Prior Art
In a conventional cam grinding machine for grinding a cam of a cam shaft, a master cam having the same cam profile as the cam to be ground is held in contact with a follower roller so as to pivot or rotate a swing table in accordance with the cam profile of the master cam. The swing table is provided with head and tail stocks to rotatably support the cam shaft, and the cam shaft is rotated synchronously with the master cam at a constant speed so as to be ground by a grinding wheel.
As shown in FIG. 1, in the grinding of a cam C, which is rotated about its own axis at a constant speed and simultaneously swung about a swing center O, by a grinding wheel G rotated at a constant speed, a circumferential speed of the cam C relative to the grinding wheel G is proportional to a radius of a circular portion, such as a top portion T or a base portion B. However, in grinding a side portion S, a grinding point P is shifted and the circumferential speed is rapidly changed, whereby the cam C is ground at its side portion S by more or less than the amount desired. Accordingly, there has been disadvantages in that error in a cam profile and an insufficient accuracy of a finished surface have been created.
In order to solve this problem, an attempt has been made, as disclosed in Japanese patent publication No. 95692/1973, wherein a servomotor for rotating a cam shaft is controlled in proportion to change in life amount or a swing amount of a cam profile of a master cam. While this attempt may be somewhat satisfactory, the circumferential speed cannot be maintained constant because the circumferential speed is not exactly proportional to the change in lift amount.